When a hot-dip aluminized steel sheet is manufactured with a continuous hot-dip aluminizing plant (line), as illustrated in FIG. 17, a base-metal steel sheet 4 is guided into a hot-dip Al--Si plating (aluminizing) bath 1 which has been adjusted to a specific bath composition and bath temperature and guided out of the bath 1 after having rounded a sink roll 2 in the bath 1. Next, the amount of the coating (the thickness of the coating layer) is adjusted by a gas-wiping unit 3 placed immediately above the bath 1. Here, the plant is generally provided with a cooling unit 5 above the bath 1 which forcedly cools the coating-metal layer (with jets of a gas, gas/liquid, etc.) so as to completely solidify the coating-metal layer before the coated steel sheet 6 reaches an upward top roll 9.
With hot-dip aluminized steel sheets manufactured in this way, diffusion of Fe atoms across the interface between the base metal steel sheet and the coating-metal layer (infiltration of Fe atoms in the base metal steel sheet into the coating-metal layer through diffusion) results in the inevitable formation of an Fe--Al--Si alloy layer at the interface. The alloy layer, being hard and fragile, promotes peeling of the coating layer from the coated steel sheet during press working. Particularly in cases where the steel sheet is subjected to strong working such as drawing or squeezing, the alloy-layer thickness must be controlled to approximately 5 .mu.m or smaller in order to ensure the press workability (e.g., Japanese Examined Patent Application Publication SHO 51-46739).
A variety of proposals have been suggested for coating conditions to control the production and the growth of the alloy-layer including:
(a) Adjustment of the coating bath so as to have a specific Al--Si bath composition (Si content: 3-13%), and limiting the bath-immersion temperature of the base metal steel sheet (the sheet temperature immediately before its immersion into the bath) to a range between the melting point of the metal in the aluminizing bath and the melting point plus 40.degree. C. (Japanese Unexamined Patent Application Disclosure HEI 4-176854); PA1 (b) Quenching of the coated steel sheet guided out of the coating bath by spraying a coolant (a liquid, gas plus liquid, etc.) from a cooling unit placed above the bath (Japanese Unexamined Patent Application Disclosure SHO 5260239); PA1 (c) Precoating of the base metal steel sheet surface with a layer of a metal having a lower melting point than the coating (i.e. plating) metal to maintain the steel sheet temperature at 500.degree. C. or lower until the coating is accomplished (Japanese Unexamined Patent Application Disclosure HEI 1-104752); PA1 (d) Setting the bath-immersion temperature of the base-metal steel sheet to a temperature 50-100.degree. C. lower than the coating bath temperature (Japanese Unexamined Patent Application Disclosure HEI 5-287488); etc.
However, it has proven difficult to satisfactorily control the alloy-layer thickness only through control of the operation conditions as suggested by the prior art, in other words through the adjustment of the coating bath composition and temperature, the control of the bath-immersion temperature of the base metal steel sheet and the high-level forced-cooling of the coated metal layer, etc. While precoating the surface of the base-metal steel sheet with a special metal layer results in an increased number of steps and an increased cost. In addition, all the processes of the prior art fail to precisely control the alloy-layer thickness, since no quantitative relationship is elucidated to exist between the production and the growth rate of the alloy layer, and the operational conditions.
After repeated thorough investigation of the phenomenon of alloy-layer production, the present inventors have found that the thickness of the alloy layer produced has a quantitative correlation with the time elapsed from the beginning of the immersion of the base-metal steel sheet into the coating bath to the completion of the solidification of the coating-metal layer on the surface of the steel sheet which has passed through the bath. Furthermore, the present inventors have discovered that adjustment of the lapsed time allows precise control of the alloy-layer thickness to a desired layer thickness (or a smaller thickness).
It has also been found that alloy layers have remarkably different section patterns depending on the operational conditions coating, that alloy layers with lower degrees of surface unevenness and thus higher degrees of flatness have higher resistance to peeling of the coating layer, that the section pattern changes depending on the time elapsed from the time at which the coated steel sheet is guided above the coating bath to the completion of solidification of the coating-metal layer, and that adjustment of the elapsed time allows control to a more desired section pattern.
The present invention, which has been accomplished based on the findings mentioned above, provides a hot-dip aluminized steel sheet with high resistance to peeling of the aluminized layer, a method of manufacturing a continuous hot-dip aluminized steel sheet which allows precise control of the thickness and the section pattern of the alloy layer produced, and an alloy-layer control apparatus which is used in the method.